To ensure the safety of the passengers and crewmembers in case of a depressurization accident or the occurrence of smoke in the aircraft, aviation regulations require on board all airliners a safety oxygen supply circuit able to supply each passenger and crewmember with an oxygen flow rate which is function of the cabin altitude.
A mask and harness system is use to provide breathing oxygen. The mask system has a face seal, a pneumatically-actuated harness, and a regulator with microphone to control the flow of oxygen and to facilitate communications in aircraft flight decks and other aircraft compartments. The system is designed for a required five-second donning with one hand. It is connected to an oxygen source and, optionally, to aircraft communications.
The usual method consists of using available pressure at the inlet of pressure regulator (which is usually closed to the harness body) in order to deploy the harness to allow the mask user to put it on his head. This method is not possible in the following cases:                when the pressure regulator is not close to the harness (for example control panel regulator in military applications),        when pressure supply is not sufficient due to, for instance, the use of On Board Oxygen Regulator.        
In those cases, to perform the task of donning the mask, a mechanical harness is usually used. However, the usual mechanical harness is heavy, bulky and expensive. A second solution consists to require the crewmember to wear his mask permanently (which can be fixed to his helmet, for instance). This second option is very uncomfortable and generates a permanent usury of the equipment for a very low probability of risk.